New and advanced transmission systems and braking systems are being developed by the automotive industry. These new systems often involve high-energy requirements. Therefore, the friction materials technology must be also developed to meet the increasing energy requirements of these advanced systems.
In particular, a new high performance, durable friction material is needed. The new friction material must be able to withstand high speeds wherein surface speeds are up to about 65 m/seconds. Also, the friction material must be able to withstand high facing lining pressures up to about 1500 psi. It is also important that the friction material be useful under limited lubrication conditions.
The friction material must be durable and have high heat resistance in order to be useful in the advanced transmission and braking systems. Not only must the friction material remain stable at high temperatures, it must also be able to rapidly dissipate the high heat that is being generated during operating conditions.
The high speeds generated during engagement and disengagement of the new transmission and braking systems mean that a friction material must be able to maintain a relatively constant friction throughout the engagement. It is important that the frictional engagement be relatively constant over a wide range of speeds and temperatures in order to minimize “shuddering” of materials during braking or the transmission system during power shift from one gear to another. It is also important that the friction material have a desired torque curve shape so that during frictional engagement the friction material is noise or “squawk” free.
In particular, transmission and torque-on-demand systems incorporate slipping clutches mainly for the fuel efficiency and driving comfort. The role of the slip clutch within these systems varies from vehicle launching devices, such as wet start clutches, to that of a torque converter clutches. According to the operating conditions, the slip clutch can be differentiated into three principle classes: (1) Low Pressure and High Slip Speed Clutch, such as wet start clutch; (2) High Pressure and Low Slip Speed Clutch, such as Converter Clutch; and (3) Extreme Low Pressure and Low Slip Speed Clutch, such as neutral to idle clutch.
The principal performance concerns for all applications of the slip clutch are the prevention of shudder and the energy management of the friction interface. The occurrence of shudder can be attributed to many factors including the friction characteristics of the friction material, the mating surface's hardness and roughness, oil film retention, lubricant chemistry and interactions, clutch operating conditions, driveline assembly and hardware alignment, and driveline contamination. The friction interface energy management is primarily concerned with controlling interface temperature and is affected by the pump capacity, oil flow path and control strategy. The friction material surface design also contributes to the efficiency of interface energy management.
Previously, asbestos fibers were included in the friction material for temperature stability. Due to health and environmental problems, asbestos is no longer being used. More recent friction materials have attempted to overcome the absence of the asbestos in the friction material by modifying impregnating paper or fiber materials with phenolic or phenolic-modified resins. These friction materials, however, do not rapidly dissipate the high heat generated, and do not have the necessary heat resistance and satisfactory high coefficient of friction performance now needed for use in the high speed systems currently being developed.
The present invention is an improvement over the Seitz U.S. Pat. No. 5,083,650 reference which involves a multi-step impregnating and curing process; i.e., a paper impregnated with a coating composition, carbon particles are placed on the paper, the coating composition in the paper is partially cured, a second coating composition is applied to the partially cured paper, and finally, both coating compositions are cured.
The present invention is an improvement over the Smith et al. U.S. Pat. No. 5,965,659 which involves the use of carbonaceous fibers derived from oxidized polyacrylonitrile based fibers.
In other friction materials, metallic fibers combined with carbon materials were included in the friction material for wear resistance. For example, Fujimaki et al. U.S. Pat. No. 4,451,590 describes a friction material having metallic fibers, filler, carbon particles, carbon fibers and phenolic resin. However, the metallic based friction materials do not have sufficient porosity and compressibility to be capable of high fluid permeation capacity during use. Also, the metallic based friction materials are not sufficiently resilient or elastic, yet resistant to compression set to be capable of withstanding high facing lining pressures of up to about 1500 psi (approximately 105 kg/cm2). The metallic based friction material also is not capable of withstanding high surface speeds of up to about 65 m/second which are generated during engagement and disengagement of the new transmission and braking systems.
The present invention is also an improvement over earlier co-owned patents by the assignee herein, BorgWarner Inc., for friction materials. In particular, U.S. Pat. No. 5,998,307 relates to a friction material having a base impregnated with a curable resin where a porous primarily layer comprises at least one fibrous material and a secondary layer of carbon particles covering at least about 3 to about 90% of the surface of the primary layer. The U.S. Pat. No. 5,858,883 relates to a base material having a primary layer of less fibrillated aramid fibers, synthetic graphite, and a filler, and a secondary layer of carbon particles on the surface of the primary layer. U.S. Pat. No. 5,856,244 relates to a friction material comprising a base impregnated with a curable resin where the primary layer comprises less fibrillated aramid fibers, synthetic graphite and filler; and a secondary layer of carbon particles and a retention aid. The U.S. Pat. No. 5,958,507 relates to a process for producing the friction material where at least one surface of the fibrous material which comprises less fibrillated aramid fibers is coated with carbon particles and a retention aid, impregnating with a phenolic or modified phenolic resin, and curing. The U.S. Pat. No. 6,001,750 relates to a friction material comprising a fibrous base material impregnated with a curable resin where the fibrous base material has a porous primary layer of less fibrillated aramid fibers, carbon particles, carbon fibers, filler material, phenolic novoloid fibers, and optionally, cotton fibers, and a secondary layer of carbon particles which cover the surface at about 3 to about 90% of the surface.
In addition, various base materials are described in commonly owned BorgWarner Inc. U.S. Pat. Nos. 5,753,356 and 5,707,905 which describe base materials comprising less fibrillated aramid fibers, synthetic graphite and filler. Another commonly owned patent, U.S. Pat. No. 6,130,176, relates to non-metallic fibrous base materials comprising less fibrillated aramid fibers, carbon fibers, carbon particles and filler.
Yet another commonly owned patent application, Ser. No. 09/707,274, now allowed, relates to a friction material having a porous primary layer and a secondary layer of silica particles covering about 3 to about 90% of the surface of the primary layer. In a preferred aspect, the friction modifying particles comprise at least one of silica particles, resin powders, carbon powder or particles and/or partially carbonized powder or particles and mixtures thereof.
In order for friction materials to be useful in “wet” applications, the friction material must have a wide variety of acceptable characteristics. The friction material must be resilient or elastic yet resistant to compression set, abrasion and stress; have high heat resistance and be able to dissipate heat quickly; and, have long lasting, stable and consistent frictional performance. If any of these characteristics are not met, optimum performance of the friction material is not achieved.
It is also important that a suitable impregnating resin be used with the fibrous base material in order to form a high-energy application friction material. The friction material must have good shear strength both when saturated with the wet resin during impregnation and when saturated with brake fluid or transmission oil during use.
It is also important, under certain applications, that the friction materials have high porosity such that there is a high fluid permeation capacity during use. Thus, it is important that the friction material not only be porous, it must also be compressible. The fluids permeated into the friction material must be capable of being squeezed or released from the friction material quickly under the pressures applied during operation of the brake or transmission, yet the friction material must not collapse. It is also important that the friction material have high thermal conductivity to also help rapidly dissipate the heat generated during operation of the brake or transmission.
As far as is known, there is no disclosure of friction material for use in transmission systems which includes a fibrous base material comprising a primary layer having deposited thereon at least a partial covering of a secondary layer of friction modifying particles that comprises a mixture of silica materials and carbon particles.
Accordingly, it is an object of the present invention to provide an improved friction material with reliable and improved properties compared to those of the prior art.
A further object of this invention is to provide friction materials with improved coefficient of friction characteristics, “compression modulus”, “hot spot” resistance, high heat resistance, high friction stability and durability, porosity, strength, and elasticity.
As a result of extensive research in view of the need for a better friction material, a friction material with improved characteristics has been developed by the invention. The present wet friction material is useful in “wet”applications where the friction material is “wetted” or impregnated with a liquid such as brake fluid or automatic transmission fluid during use. During use of the “wet” friction material, the fluid is ultimately squeezed from or is impregnating the friction material. Wet friction materials differ greatly, both in their compositions and physical characteristics from “dry” friction materials.